It is generally known that a low operating temperature is necessary in order to increase the efficiency and the long-term stability or lifetime of photovoltaic cells. Thus, excess heat generated in the cell by irradiation not converted into electrical energy needs to be extracted. This extraction can be carried out by passive or active cooling means, wherein active cooling can be achieved, for example, by dynamic contact with a cooling fluid, and passive cooling can be achieved by attaching the cell to a heat sink.
Die-attach or die-bonding is the process of attaching a semiconductor die or chip to a substrate, a package, or another die or chip. For a photovoltaic cell assembly, this corresponds, in particular, to the process of attaching the solar cell to a heat sink, which can be used for passive cooling as mentioned above. It is generally known that this attachment can be carried out by soldering, welding, adhesive bonding, sintering and the like, and that the quality of the attachment layer obtained by either of these methods is important for efficiently transferring waste heat from the photovoltaic cell to the heat sink.
The quality of the die-attach is, therefore, a crucial parameter for characterizing photovoltaic cell assemblies, in particular, in view of efficiently discarding defective photovoltaic cell assemblies, meaning assemblies for which the die-attach fails in efficiently conveying heat from the cell to the heat sink, prior to the assembly of photovoltaic modules comprising a frame, and one or more photovoltaic cell assemblies. This is even more important for concentrator photovoltaic modules comprising corresponding lenses for focusing light on the cells, as these assemblies are expensive, and the replacement of a defective photovoltaic cell assembly therein is complex and costly.
It is generally known to use thermal transient testing to verify the die-attach of an LED to its mounting substrate. Such methods take advantage of the linear nature of the temperature coefficient of the LED material being tested and the fact that the thermal mass of the LED is several orders of magnitude less than that of the mounting substrate. The forward voltage of the LED is first measured using a small low-heating measurement current, which is chosen low so as to essentially not introduce any negligible heating in the LED device. The LED is then subjected to a short non-destructive heating current. Quickly after removal of the heating current, the measurement current is re-applied and the forward voltage is re-measured and compared to the value measured before the heating pulse. The difference in forward voltage before and after application of the heating current and the temperature coefficient of the LED material are then used to determine the temperature rise of the LED above the mounting substrate temperature. A time-consuming temperature calibration on the measurement points (which are stabilized in temperature) by an external temperature sensor allows the calibration of such measurements to absolute values of the thermal resistance.
A method of thermal transient testing for characterizing an already fully assembled solar module under operating conditions is disclosed in B. Plesz, et al., 2011 (Characterization of solar cells by thermal transient testing; Proceedings of the 17th International Workshop on Thermal investigations of ICs and Systems, THERMINIC 2011, held on 27-29 Sep. 2011 in Paris, France). This document discloses, in particular, a method for testing and characterizing the quality of the die-attach in a concentrator photovoltaic module, which is used for the quality management of the fully assembled module or for testing the module during operating conditions.
However, further to being performed only on a fully assembled photovoltaic module and characterizing the same under operating conditions, especially in the case of concentrator photovoltaic modules, the method of B. Plesz, et al., 2011, requires measurements of absolute values of the thermal resistance (Rth), and a temperature-sensitive calibration, which needs to be performed under dark conditions, in other words, when the solar module is not being irradiated. In particular, the temperature-sensitive parameter value has to be calibrated for each device under test to measure the temperature dependency of the forward voltage of the diode. The method further uses a constant sensor current driven through the diode, while the forward voltage values are measured at selected temperature-sensitive points.
Thus, the method disclosed in B. Plesz, et al., 2011, like other known methods of die-attach testing, is too time consuming and too complex to be integrated in a manufacturing line, in particular, prior to assembling the photovoltaic modules. In other words, this testing method cannot be integrated or automated as a part of the manufacturing process of photovoltaic cell assemblies and can only be implemented at a later stage, on a sampling basis, for instance, for device characterization during the operation of the already fully assembled photovoltaic cell assembly.
As a consequence, the method disclosed in B. Plesz, et al., 2011, is not usable to prevent the assembly of full photovoltaic modules comprising one or more defective photovoltaic cell assemblies.
In other words, there is a strong need in the photovoltaic industry for a suitable method for efficiently monitoring the die-attach quality in a photovoltaic cell assembly. In particular, there is a need for a method of inline monitoring that allows discarding nonviable photovoltaic cell assemblies immediately, preferably prior to being conveyed to the assembly lines for assembling photovoltaic modules.